VOPc on Graphene

The majority of the electronics that we use are made from inorganic materials that are often hard to recycle, include dangerous elements like lead, and sometimes include rare elements that are expensive. This combines to increase both the financial and environmental cost of the vast electronics industry.

One alternative is to use organic molecules. These are molecules that are mostly made from carbon, and, in contrast to inorganics, they are often cheap, disposable, and easy to process. Organic molecules, like one named vanadyl phthalocyanine (or VOPc), could be used to make cheaper, more environmentally friendly solar cells and transistors. But the best attempts of using organic materials have yet to surpass their inorganic counterparts. The performance of organics lies in how the molecules stick together to form semiconducting crystals.

In this paper, we looked at the growth of VOPc crystals on graphene. Graphene is chosen as a growth substrate for two key reasons. One, is that it is an excellent conductor that is almost completely transparent to visible light, and so could be used in solar cells and LEDs. The second reason is that it is also virtually transparent to electrons. This allows us to deposit VOPc onto freely suspended graphene, and use the latest transmission electron microscopes to study how the VOPc crystals grow. This is something you cannot do with traditional substrates like glass.

We made three key findings. The first is that heating the graphene substrate during the VOPc deposition causes large crystals to grow. This is likely because molecules move around faster on a hot substrate and so are more likely to find and stick to an already-existing VOPc island than to make their own. This allows crystals to grow larger, and larger crystals generally perform better than smaller ones in devices.

Atomic force microscopy (AFM) from graphene on copper after VOPc deposition at a) room temperature, b) 75 °C, c) 125 °C, d) 155 °C, and e) 175 °C. The graph in f) shows how the islands get bigger but the coverage drops as the graphene gets hotter during deposition.

The second finding is that the VOPc crystals do not necessarily align with the graphene crystal structure underneath. This lack of epitaxy (a term used to describe how one layer can influence the orientation of the layer above) is quite important, as epitaxy is often used to increase crystal size. These results show, then, that it is not essential to rely on epitaxy to grow large crystals.

The third result is that the large-crystal behaviour seen for graphene is not seen in graphene oxide (GO). GO is a much cheaper, solution-based form of graphene that is easier to process, and so is also being carefully considered for large-scale applications. Its structure is a graphene sheet with oxygen groups attached, and it is these oxygen groups that disrupt the large crystal growth: the VOPc molecules are more likely to nucleate at these groups than reach the already-existing islands.

Overall, the results show that graphene could be an ideal substrate for organic molecular crystal growth because it enables large crystal growth, without the need to rely on epitaxy. However, the graphene should be clean and flat; any disruption (like the oxygen groups in graphene oxide) can hinder the growth.